Control of scale formation in seamless tubes in expanding mills



United States Patent 3 148 445 CONTROL OF SCALE FOiZli/IATIGN 1N SEAMLESS TUBES IN EXPANDING BELLS Howard H. Nichols and John D. Keller, Pittsburgh, Pa., assignors to Hazen Engineering Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Dec. 6, 1961, Ser. No. 157,595 2 Claims. (Cl. 29-523) This invention relates to new and useful improvements in method of piercing and expanding seamless tubing and more particularly to a method of correcting bad scale conditions existing on the inner wall of the tube after it is pierced from a billet and before it is subjected to the expanding operation.

It is among the objects of this invention to provide a method of producing a controlled scale on the inner wall of a pierced tube during the reheating step and breaking it off when expanding the tube with a mandril traveling inside the tube with driving rolls on the outside of the tube.

Heretofore the improperly formed scale was rolled into the inner surface of the tube during the expanding proc ess and it is a primary object of the invention to provide a scale of suitable character that will break off easily in the expanding mill.

In accordance with the present invention, we propose to charge the interior of a pierced billet or tube with oxygen or other scale forming gases While it is being reheated preliminary to expanding the same.

The billets are first preheated in a billet heating furnace or a rotary hearth furnace to a tem erature approximating 2300 F. and the heated billets are then sent to the piercing mill to pierce and form a tube which is discharged from the piercer onto a conveyor where it will travel to the charging machine in front of a heating furnace for reheating the pierced tube prior to being charged into an expanding mill.

The tubes coming from the piercing mill may have an inside diameter of and be approximately 45 long. After leaving the piercing mill and as they enter the reheating furnace, they are at a temperature of approximately 1000 F. and by virtue of their travel on the conveyor from the piercing mill to the entrance of the reheat furnace, the inside of the tube is filled with air. The charging machine embodies a long rod that pushes the pierced tubes endwise into the reheating furnace and the air inside of the pipe being 1000 F. is at lower temperature than the furnace which is at approximately 2100 F. The furnace temperature is held constant and the furnace has three to five lengths of tubes charged at one time. From the charging end of the furnace the tubes are rolled down an inclined hearth to the opposite side of the furnace where they are discharged in the same direction as they are charged.

Because the tubes are heated primarily by radiation from the furnace roof, floor and walls by very low velocity flames, little or none of the furnace atmosphere enters the inside of the tubes. Consequently, the scale formed on the inside of the tube is formed by the oxygen in the atmosphere carried in from the conveyor on the charging side, which contains sufiicient oxygen to form a layer of FeO .0034 inch thick. It can be realized, however, that the total inner surface of the tube does not reach scaling temperature at the same time. Consequently, the portion of the tube reaching scaling temperatures first will have a heavier jacket than the portion of the tube more slowly heated.

When the reheated tube is discharged to the expanding mill, a mandril travels inside the tube and driving rolls over the outside of the tubing. The intense pressure put on the walls of the tube expand it in diameter and during this expanding process, the improperly formed scale is rolled into the inner surface of the tube. The problem is to properly form the scale inside the tube of uniform thickness and of a character that it will break off easily in the expanding mill. We have discovered that by preferably preheating oxygen to approximately 800 to 900 and charging it into the tube as it is being charged into the reheating furnace, a 15" diameter pipe 45 long will take approximately 4 pounds of oxygen to form a proper scale on the inner surface of the tube. It will form scale to Fe 304 or Fe 203, either one of which is satisfactory to cause the scale to be easily removed by the expanding mill mandril and this scale cannot possibly be rolled into the inner wall of the tube. The furnace temperature being 2100, the gases inside the furnace will be approximately one-half the weight of the 1000 oxygen that is charged into the tube. The oxygen may be preheated by a standard metallic recuperator and it will not be necessary to plug either end of the tube to keep the oxygen from flowing out. The oxygen may be charged into the tubing by an oxygen lance which can be Withdrawn before it reaches the critical metal temperatures.

Although one embodiment of the invention has been herein illustrated and described, it will be evident to those skilled in the art that various modifications may be made in the details of construction without departing from the principles herein set forth.

We claim:

1. In the method of making seamless steel tubing with the inner wall free of scale, the steps of:

(a) heating a steel billet to a temperature of approximately (2300 R),

(15) passing the heated billet through a piercing mill to form a hollow tube,

(0) charging the tube into a reheating furnace having a temperature of approximately (2100 F.), and

(d) simultaneously charging the interior of the tube with oxygen in excess of the oxygen normally contained in the atmosphere to form a scale of substantially uniform thickness on the inner wall thereof, and,

(e) passing the reheated tube through an expanding mill to increase its diameter and simultaneously break off the scale on the inner wall of the tube.

2. A method of making seamless steel tubing as set forth in claim 1 in which the oxygen charged into the interior of the tube is preheated to a temperature of from 800 F. to 1000 F.

References Cited in the file of this patent UNITED STATES PATENTS 2,363,476 Bannister Nov. 28, 1944 2,442,485 Cook June 1, 1948 OTHER REFERENCES The Making, Shaping and Treating of Steel, US. Steel, 7th edition, 1957, pp. 742, 743.

The Metals Handbook, A.S.M., 8th edition, 1961, p. 731. 

1. IN THE METHOD OF MAKING SEAMLESS STEEL TUBING WITH THE INNER WALL FREE OF SCALE, THE STEPS OF: (A) HEATING A STEEL BILLET TO A TEMPERATURE OF APPROXIMATELY (2300* F.), (B) PASSING THE HEATED BILLET THROUGH A PIERCING MILL TO FORM A HOLLOW TUBE, (C) CHARGING THE TUBE INTO A REHEATING FURNACE HAVING A TEMPERATURE OF APPROXIMATELY (2100* F.), AND (D) SIMULTANEOUSLY CHARGING THE INTERIOR OF THE TUBE WITH OXYGEN IN EXCESS OF THE OXYGEN NORMALLY CONTAINED IN THE ATMOSPHERE TO FORM A SCALE OF SUBSTANTIALLY UNIFORM THICKNESS ON THE INNER WALL THEREOF, AND, (E) PASSING THE REHEATED TUBE THROUGH AN EXPANDING MILL TO INCREASE ITS DIAMETER AND SIMULTANEOUSLY BREAK OFF THE SCALE ON THE INNER WALL OF THE TUBE. 